1. Field of the Invention
The present invention relates generally to the preparation of conductive fibers, and more particularly to the preparation of fibers containing intrinsically conductive polymers.
2. Description of the Prior Art
Synthetic fibers are widely used in the textile industry and are increasingly being used outside the classical textile fields in novel applications such as fibers for reinforcing thermoplastics and duroplastics used in manufacturing automobiles, airplanes and buildings; optical fibers for light telephony; and fibrous materials for numerous medical applications. This diverse application of synthetic fibers is largely based on the development of techniques for "tailor-making" fibers to provide physical properties that are desirable for a particular use.
When used in textiles, for example, it is often desirable that synthetic fibers have low resistivity, or an electrical conductivity sufficient to dissipate static electrical charge. This would reduce or prevent the development of static electricity, which causes fabrics comprised of such fibers to cling and to be difficult to clean. However, some of the most important synthetic fibers, particularly nylon, polyester, and acrylic fibers, have low electrical conductivity. Thus, the development of methods for increasing the electrical conductivity of synthetic fibers is an area of active research in the textile industry.
For example, techniques suggested to increase conductivity in polyester fibers include dispersing fibrils comprised of a hydrophilic or conductive polymer in the polyester matrix, forming sheath-core bicomponent fibers with a polymer containing conductive carbon black or a metal oxide in the sheath or in the core, and metallizing or graphitizing the fiber surface. See, e.g., J. E. McIntrye, Polyester Fibers, in Fiber Chemistry, 40-41, 1-71 (Menachim Lewin & Eli M. Pearce eds., 1985), incorporated herein by reference.
Reported methods for making electrically conductive acrylic fibers include incorporating carbon black into the fibers during the spinning process and treating spun fibers with zinc oxide or copper ions. See, e.g., Bruce G. Frushour & Raymond S. Knorr, Acrylic Fibers, in Fiber Chemistry, 341-342, 171-370 (Menachim Lewin & Eli M. Pearce eds., 1985), incorporated herein by reference.
The above methods involving carbon black produce fibers of limited use in that they are black or grey. Moreover, many composite fibers containing metal oxide have poor durability when used in textiles.
In addition, some groups have recently tried incorporating an intrinsically, conductive polymer into synthetic fibers to improve their electrical conductivity. An intrinsically conductive polymer (ICP) is an organic polymer which has a poly-conjugated .pi.-electron system such as double or triple bonds, or aromatic or heteroaromatic rings. For a review, see Conjugated Polymers and Related Materials (W. R. Salaneck et al. eds., Oxford University Press 1993), incorporated herein by references. Sometimes referred to as "synthetic metals", intrinsically conductive polymers (ICP's) are completely different from "conducting polymers" which are physical mixtures of a nonconductive polymer with a conducting material such as a metal or carbon powder distributed throughout the material.
An ICP may exist in various electrochemical forms which can generally be reversibly converted into one another by electrochemical reactions such as oxidation, reduction, acid/alkali reactions or complexing. These reactions are also referred to in the literature as "doping" or "compensation". At least one of the possible electrochemical forms of an ICP is as a very good conductor of electricity, e.g., has a conductivity of more than 1 S/cm (in pure form). Electrically conductive forms of an ICP are generally regarded as polyradical cationic or anionic salts.
Although ICP's have a number of potential uses, their conductive properties make ICP's a desirable component of fibers for use in textiles, carpets and other commercial applications.
For example, U.S. Pat. No. 5,423,956 to White et al. discloses a process for making composite polymer fibers in which a coating of a conductive organic polymer is electrochemically formed on the outer surface of a polymeric fiber. Similarly, polyaniline with a counterion doping agent has been polymerized onto the surface of a fiber or fabric material. (See U.S. Pat. No. 4,803,096 to Kuhn et al., incorporated herein by reference.) These and other processes which polymerize polyaniline on the surface of fibers, or textiles, are unsatisfactory in that they require an additional manufacturing step which, besides adding cost to the product, adds significant technical problems in the control and operation of such processes.
In addition, several methods of preparing fibers containing the intrinsically conductive form of polyaniline have recently been reported. Andreatta and coworkers report a method of producing fibers of polyaniline from a solution in concentrated sulfuric acid (Andreatta et al., 26 Synth. Met. 383-389 (1988), incorporated herein by reference). However, fibers composed entirely of polyaniline are often brittle and inflexible and thus not suitable for use in textiles or carpets.
High molecular weight polyaniline has also been spun into fibers from the nonconductive form dissolved in N-methyl pyrrolidone followed by subsequent doping of the fibers with HCl to produce the conductive form of polyaniline. (See, for example, U.S. Pat. No. 5,312,686 to MacDiarmid et al., incorporated herein by reference.) This and other approaches which add dopants after formation of the fiber form fibers in which the conductivity is of limited durability in that they usually require that small dopant molecules be used so that doping time will not be prohibitively long. However, these low molecular weight dopants can diffuse out of a fiber when it is washed or heated, leaving the fiber undoped, i.e., nonconductive.
It has also been proposed to use ICP's such as polyaniline as an additive in fibers spun from molten polymers such as polypropylene and Nylon. An inherent barrier to the use of ICP's as an additive in melt-spun fibers is their thermal instability at the temperatures required for melt-spinning.
Another approach is described in U.S. Pat. No. 5,248,554 to Hsu, in which filaments of p-aramid yarns are impregnated with a polyaniline by passing the yarn through a solution of polyaniline in sulfuric acid. The sulfuric acid causes the fiber to swell and ultimately causes longitudinal cracks in the fiber, allowing the polyaniline to penetrate into the fiber. The polyaniline may be undoped, thus requiring subsequent doping to enhance conductivity, or the polyaniline may be a sulfonated polyaniline that does not require subsequent doping. However, impregnation of p-aramid filaments with polyaniline in sulfuric acid requires careful control of the concentration and time of exposure to the sulfuric acid to avoid excessive cracking of the filaments and loss of tensile properties. Moreover, unless rendered insoluble by heat treatment of the fiber, the impregnated sulfonated polyaniline is somewhat soluble in 0.1 M ammonium hydroxide.
Despite the previous efforts to incorporate protonated, or doped polyaniline into fibers which have properties suitable for commercial use, the described processes are either complicated and/or the conductivity of the fibers produced is of limited durability. Thus, there continues to be a need for incorporating ICP's into fibers formed from any a variety of polymers, copolymers, or polymer blends using standard fiber manufacturing processes to produce fibers which exhibit conductivity in a dry environment even after repeated flexing and washing.